As wireless Network Access Points (NAPs) are becoming ubiquitous, it is only a logical consequence that many mobile devices support more than one RAT to offer an increased connectivity. In this regard, US 2007/0173283 A1 describes an exemplary mobile device supporting two different RATs. Specifically, the mobile device comprises a first module providing network access according to the Universal Mobile Telecommunications System (UMTS) specifications and a second module network providing access to a Wireless Local Area Network (WLAN).
Such multi-RAT devices are typically designed to permit a handover between the internal RATs (IRAT handover). IRAT handovers may, for example, be performed in case coverage of one RAT is fading or another RAT with better Quality of Service (QoS) parameters becomes locally available. For the coordination of IRAT handovers, in US 2007/0173283 A1 a mobility middleware function is proposed. The mobility middleware function comprises an IRAT handover functionality that performs seamless handover between the UMTS network and the WLAN with the purpose of providing service continuity to a user.
Multi-RAT devices as those discussed in US 2007/0173283 A1 suffer from the drawbacks that a seamless handover typically requires a data connection between the RAT modules and an associated control mechanism for inter-module data transfer. Such an inter-module data connection is required for the following reason. When a handover from a first RAT supported by a first RAT module to a second RAT supported by a second RAT module is in progress, the uplink data from a user application are during the handover phase still sent to the first RAT module. Has the handover to the second RAT been successful, the first RAT module will thus typically still have user data in its local buffers that has not been transmitted. This user data will have to be transferred via the data connection to the second RAT module for transmission. Otherwise, i.e., if the user data buffered by the first RAT module is simply discarded, problems on the network side (which may still be expecting the buffered user data) and on the side of the mobile device (e.g., an instability of the user application that is expecting response data from the network) may occur.
Obviously, the data transfer between the RAT modules in connection with a seamless handover requires additional data interfaces and additional control mechanisms to align the data stream towards the network. Such interfaces and control mechanisms lead to additional costs. Moreover, it might be difficult to harmonize the inter-module data transfer in case the involved RAT modules come from different manufactures with proprietary interfaces and proprietary control mechanisms.
Similar problems arise in case the first RAT module becomes temporarily disconnected from the network while the second RAT module is not (or not yet) connected. In such a case the user application may continue sending user data towards the buffers of the first RAT module, while it is uncertain whether the first RAT module will become connected again or whether the second RAT module will become connected instead (e.g., because first RAT coverage got lost).